Electrochemical batteries are frequently used to store energy in a wide variety of applications. Amongst those applications, several call for high-voltage storage such as in UPS (Uninterruptible Power Supplies) and transport applications. In this last field of application, total voltage range may vary from 24 to 42V or even well above 500V. Since most of the electrochemical technologies in use or in development today have typical cell voltages between 1 and 5 Volts, several cells have to be connected in series to achieve the desired total output voltage required. For example, locomotives were built by the Applicant using a string of 336 cells of 2V to reach a nominal voltage of 672V. Use of high voltage supply system is frequently dictated by load constraints that do not always enable charging and discharging the battery pack at recommended low and high voltage levels that would maximize the battery life.
When used in long series strings, severe problems may occur if one cell deteriorates more rapidly than others. In this condition, the battery pack is able to supply large currents in a given load while one cell may have a higher internal resistance leading to overheat and possible fire or explosion. Mitigation techniques used on an industrial basis consisted mostly in measuring individual cell voltages and/or temperature in operation to detect failing devices and disable high-power operation. Measurement of cell resistivity, mostly on specific surveys, is more complex but also informative of the components health.
It is generally recognized that battery cell longevity benefits from frequent current circulation. Tests performed on cells from which a constant small current is drawn show better consistency with time than cells at rest. However, draining constant current dissipates power and requires more frequent recharges, which goes against the purpose of using batteries for energy storage. Another aspect of battery longevity relates to minimization of sulfation deterioration by pulsed current through the battery, in charge or discharge modes.
Security constraints (IEEE 1491) states that the battery monitor design should be such that a catastrophic failure to the monitor should not have any effect on the battery system, critical load, or personnel in the immediate area. This suggests that any battery management or equalizer system would preferably be disconnected from the series string if the string becomes broken at any point. As the equalizer circuit is concerned, it is advantageous that each battery to equalizer circuit be galvanically isolated from other battery circuits and, that upon detection of a broken battery pack, the circuit would still operate with possible reduced performance at least in order to be able to report the fault.